1. Field of the Invention
The present invention relates to a tilt type steering system utilized for adjusting a height position of a steering wheel to match with a physique of the driver.
2. Related Background Art
As disclosed in, e.g., Japanese Utility Model Laid-Open Application Nos. 61-199472 and 2-3972 (corresponding to U.S. Pat. No. 4,901,592), a conventionally known system called a tile type steering wheel so that the height of the steering wheel is variable in accordance with a physique of the driver and a driving posture.
The known tilt type steering system is, as illustrated in FIGS. 1 and 2, constructed such that a steering wheel 1 is fixed to an upper end (rear end) of a steering shaft 2 which rotates in a torsional direction by operating this steering wheel 1; the steering shaft is inserted into a steering column 3; upper and lower (front and rear) ends of the steering column 3 are supported by corresponding portions of a car body B; and the vertical position of the upper support member on the upper side (rear side) is made freely adjustable.
The steering column 3 is constructed by combining a small-diameter inner column 4 with a larger-diameter outer column 5 in series. The outer column 5 makes a displacement with respect to the inner column 4 only when a strong force acting in an axial direction is exerted on the steering column 3, whereby an entire length of the steering column 3 is shortened.
The lower end (front end) of the above-described steering column 3 is pivotally supported on the car body through a horizontal shaft 6. On the other hand, a fixed bracket 7 fixed to the car body (on the lower side of a dash board) has side portions 7a, 7b formed with slits 8, 8 each assuming a circular arc configuration in vertical directions, with the horizontal shaft being centered At the upper part of the steering column 3, a lift bracket 9 is fixed to a portion sandwiched in this fixed bracket 7. A single piece adjust bolt 11 is inserted into the slits 8, 8 as well as into elongate U-shaped notches 10, 10 formed in sides 9a, 9b of lift bracket 9 and opened at the rear edge of the lift bracket 9. This adjust bolt 11 is also inserted within of a bridging pipe provided to span the space between two side walls of the lift bracket 9 so as not to shrink a gap between the two side walls when, as will be mentioned later, tightly fastening the adjust bolt 11 and an adjust nut 14.
Flat members 13 parallel with each other and defined as a rotation preventing mechanism are formed at a head part 12 provided at the proximal end (right end in FIG. 2) of adjust bolt 11. Simultaneously, flat member 13 engages with a side edge of one slit 8 (right side in FIG. 2), thereby preventing the rotation of the adjust bolt 11. Then, the adjust nut 14 is screwed to a part protruded from the outer surface of the fixed bracket 7 at the tip of the adjust bolt 11. At the same time, the proximal end of a tilt lever 15 is connectively fixed to this adjust nut 14. The adjust nut 14 screwed to the adjust bolt 11 is freely fastened and released from fastening by manipulating this tilt lever 15.
By the above construction, by manipulating the tilt lever to release (loosen) the adjust nut 14, it becomes possible to move the adjust bolt 11 along the slits 8, 8 of the fixed bracket 7 so as to adjust the position of the steering column 3, and thus the height of the steering wheel 1. Thereafter, the adjust nut 14 screwed to adjust bolt 11 can be fastened by means of the tile lever 15 to fix the steering column with the steering wheel at the desired height.
For example, the adjust nut 14 may be tightly fastened in such a state that the adjust bolt 11 is moved up to the upper ends of the slits 8, 8, resulting in a state where the steering wheel 1 is raised. The adjust nut 14 may also be tightly fastened in such a state that the adjust bolt is moved to the lower ends of the slits 8, 8, resulting in a state where the steering wheel 1 is lowered.
In the tilt type steering system constructed and functioning in the way described above, for the purpose of not only connectively fixing the proximal end of the tilt lever 15 to the adjust nut 14 but also causing, as will be stated later, no fluctuation in release load of the lift bracket 9, it is required that these two members 15, 14 be inseparable.
The reason for this is that it is necessary to maintain a force for clenching the fixed bracket 7 and the lift bracket 9 between adjust nut 14 and the head part 12 of the adjust bolt 11 at a designed value, when the adjust nut 14 is tightly fastened by the tilt lever 15 to fix a height position of the steering wheel 1.
If the tilt lever 15 and the adjust nut 14 are disassembled and reassembled, a positional relation between the two members 15, 14 changes. This can change the force by which the adjust nut 14 and the head part 12 clench the fixed bracket 7 and the lift bracket 9 when the tilt lever is manipulated to tighten the adjust nut 14.
If this force is too small, the function to hold the steering wheel 1 in the predetermined height position becomes impaired. Whereas if too large, in the event of an accident of collision, there is a possibility of increasing an impact exerted on the body of the driver when colliding with the steering wheel 1 in a construction of the type in which the lift bracket 9 releases, as will now be explained.
In the event of an accident, subsequently to a so-called primary collision in which the automobile collides with other automobiles, there occurs a so-called secondary collision in which the driver's body collides with the steering wheel 1. On the occasion of the secondary collision, for relieving the impact exerted on the driver's body, as disclosed in, e.g., Japanese Utility Model Laid-Open Application No. 2-3972, there is adopted a so-called collapsible structure in which the entire length of the steering, column 3 is shortened when an intense impact force is exerted in the axial direction. It is simultaneously practiced that the lift bracket 9 fixed to this steering column 3 is so supported inwardly of the fixed bracket 7 fixed to the car body as to be releasable when receiving the intense impact force.
When such an impact absorbing structure is taken, it is required that the lift bracket 9 be releasable from the interior of the fixed bracket 7 on the basis of a predetermined impact force. If the above-described force, increases due to the disassembly and the reassembly of the adjust nut 14 and the tilt lever 15, however, even when the predetermined impact force is applied to the steering wheel 1, the lift bracket 9 won't release from the fixed bracket 7, and it follows that the large impact is exerted on the driver's body because of the secondary collision.
For this reason, a magnitude of the force explained above in the case of rotating the tilt lever 15 up to the predetermined position has hitherto been adjusted in the factory. Simultaneously, as depicted in FIG. 2, the adjust nut 14 is welded to the tilt lever 15, thus, making the two members 14, 15 inseparable. Alternatively, a part of the adjust nut 14 is plastically deformed, thus making the two members 14, 15 inseparable.
However, the operation to weld the adjust nut 14 and the tilt lever 15 is troublesome. Further, the partial plastic deformation of the adjust nut 14 requires a dedicated equipment, and so on. In any case, those operations unfavorably cause an increment in the manufacturing costs of the tilt type steering system.